In a vehicle mounting an automatic transmission that comprises a fluid transmission, the vehicle has a structure that a hydraulic pressure is produced by a hydraulic pump driven by an engine, and a multiple-disc clutch is engaged by the hydraulic pressure to secure a gear position. When the engine is brought into a halt, therefore, no hydraulic pressure can be produced and consequently, the transmission automatically becomes a neutral state at the time when the engine is brought into a halt. Therefore, the vehicle mounting the automatic transmission that comprises a fluid transmission is not capable of limiting the rotation of the wheels by utilizing the frictional force of the engine. Therefore, the vehicle mounting the automatic transmission that comprises a fluid transmission has a parking gear that is provided on the output shaft of the transmission and is equipped with a parking lock mechanism, which selectively engages with the parking gear, so that the parking lock mechanism is engaged with the parking gear at the time of parking. When the parking gear provided on the output shaft of the transmission is locked, however, a maximum output torque of the transmission acts on the parking lock mechanism creating a condition, which is very disadvantageous from the standpoint of strength. In the medium- to heavy-sized trucks mounting the automatic transmissions, therefore, the parking lock mechanism is not established from the standpoint of strength and hence, the parking lock mechanism is hardly furnished.
Meanwhile, in the vehicles using a parallel shaft gear type transmission and using a hydraulic multiple-disc clutch or an electromagnetic powder clutch, too, the clutch is disconnected when the engine is brought into a halt and hence, the rotation of the wheels cannot be limited by utilizing the frictional force of the engine. In the vehicles of this type, too, therefore, it can be contrived to provide the output shaft of the transmission with a parking gear and to lock the parking gear at the time of parking. In the medium- to heavy-sized trucks, however, the parking lock mechanism is not established from the standpoint of strength as described above. To solve this problem, JP-A 2000-264178 discloses a parking device for a vehicle having a parking gear provided on the input shaft of the transmission so that the parking gear is locked at the time of parking.
In the parking device described in the above publication, the parking lock mechanism is operated by turning the parking switch on to lock the input shaft of the transmission at the time of parking of the vehicle. After the input shaft of the transmission is locked by the parking lock mechanism, then, the manual transmission is engaged with a predetermined gear position to couple the input shaft of the transmission to the output shaft thereof to limit the rotation of the wheels. However, if the input shaft of the transmission is locked, the gears cannot be engaged unless phases of the teeth of the synchromesh device are in agreement at the time of bringing the manual transmission into engagement with a predetermined gear position. As a result, there arouses a problem in that even if the input shaft is locked, the transmission is placed in the neutral state and hence, the rotation of the wheels is not limited.